Coatings of tin, lead, bismuth, and alloys thereof have been applied to surfaces of copper and copper-based alloys by displacement plating such as by immersion plating techniques. Chemical displacement plating has been used in the manufacture of printed circuitboards (PCB's) and particularly multilayer printed circuitboards. Printed circuitboards generally comprise a non-conducting or dielectric layer such as a fiberglass/epoxy sheet which is clad with a metal conductive layer such as copper on one or both surfaces. The metal layer on the PCB, before processing, typically is a continuous layer of copper which may be interrupted by a pattern of plated through holes linking both surfaces of the board. During processing, selected portions of the copper layer are removed to form a raised copper circuit image pattern. Multilayer PCB's are typically constructed by inter-leaving imaged conductive layers such as one containing copper with dielectric adhesive layers such as a partially cured B-stage resin, i.e., a prepreg, into a multilayer sandwich which is then bonded together by applying heat and pressure. Production of these types of PCB's is described in "Printed Circuits Handbook," Third Edition, Edited by C. F. Coombs, Jr., McGraw-Hill, 1988, which is incorporated herein by reference. Since the conductive layer with a smooth copper surface does not bond well to the prepreg, copper surface treatments have been developed to increase the bond strength between the layers of the multilayer PCB sandwich. One example of a copper surface treatment is the use of immersion tin and tin alloys as a bonding medium for multilayer circuits as disclosed by Holtzman et al (U.S. Pat. Nos. 4,715,894 and 4,882,202). In these patents, an immersion tin composition is disclosed containing thiourea compounds and urea compounds to displacement plate the copper surface of each PCB with tin by the immersion process prior to laminating them to form a multilayer board.
U.S. Pat. No. 5,196,053 (Dodd et al) describes displacement tin plating, and more particularly, the use of a complexing agent which is an imidazole-2-thione compound. This complexing agent is utilized in place of thiourea which is described in the prior art. More particularly, U.S. Pat. No. 5,196,053 describes an aqueous plating solution for displacement plating of a substrate metal surface with another metal, and the plating solution comprises
(i) a metal ion of a free metal, wherein the free metal is different from the metal of the substrate surface; PA1 (ii) a complexing agent which is an imidazole-2-thione compound; and PA1 (iii) an acid. The acids that may be used include organic acids and inorganic acids based on sulfur, phosphorus, halogens, or mixtures thereof. The sulfur-based mineral acids are preferred, and examples of these include sulfuric acid and sulfamic acid. A mixture of sulfuric and hypophosphorous acids is particularly preferred. Examples of organic acids that can be used include monocarboxylic or dicarboxylic acids having up to about 6 carbon atoms such as formic acid, acetic acid, malic acid, maleic acid, etc. PA1 (A) at least one bath-soluble metal salt selected from the group consisting of stannous salts, lead salts, bismuth salts, indium salts, gallium salts, and germanium salts; PA1 (B) at least one complexing agent selected from the group consisting of thiourea compounds and imidazole thiones, and p1 (C) at least one amidine. Preferably, the aqueous plating baths also contain at least one acid, and the baths optionally may contain one or more surfactants. The incorporation of the amidine into the plating baths results in a solution exhibiting desirable plating characteristics and increased copper holding power.
U.S. Pat. No. 5,160,422 (Nishimura et al) relates to plating baths for immersion tin-lead plating on copper or copper alloys. In addition to tin and lead, these baths contain an organic sulfo compound such as an alkane sulfonic acid or a hydroxy alkane sulfonic acid, and thiourea. Fluoborates also may be included in these plating solutions.
U.S. Pat. No. 5,554,211 describes plating baths for depositing tin, lead, bismuth, indium, gallium and/or germanium onto metal surfaces wherein the baths contain metal salts of at least one of the named metals, at least one acid selected from fluoboric acid, alkane sulfonic acids and alkanol sulfonic acids, and a complexing agent which is an imidazoline-2-thione. Optionally the baths can contain one or more surfactants.